The vertebrate hindbrain is essential for controlling an array of behaviors, from voluntary movements of the craniofacial musculature to autonomic functions of the cardiovascular and gastrointestinal systems. These behaviors rely on the precise registration of motor neurons with their peripheral targets along the head and body's anterior-posterior (AP) axis. This highly ordered relationship originates from a simple embryonic body plan in which motor neurons develop within individual rhombomeres and their prospective targets in adjacent branchial arch tissues. A major cellular contribution to this motor neuron-peripheral target relationship ;omes from the neural crest cell, a restricted stem cell population that arises from the dorsal rhombomere and migrates into the surrounding branchial arch tissue. The positional information imposed upon the varied cell types constituting this motor neuron circuit is largely provided by the AP-restricted expression of the Hox genes. However, the mechanism that maintains the AP-restricted expression of the Hox genes and their ability to control the differentiation of the neurons derived from the hindbrain and the neural crest cell remain to be defined. In the first aim, we will use a genetic fate map of the rhombomeres to identify the neuronal lineages that arise from neural crest cells and their possible regulation by the Hox genes. In the second aim, we will address the role of Hox genes in neuronal differentiation through the use of a conditional mutagenesis system to disrupt Hox gene function among progenitors and postmitotic motor neurons in the ventral neural tube. In the third aim, we will explore a mechanism by which Fgf signaling regulates motor neuron-subtype identity by repressing the activity of the Hox genes in the hindbrain. The latter aim may reveal a mechanism that establishes the different motor neuron identities along the entire AP axis of the central nervous system. An understanding of the molecular and cellular determinants contributing to the formation of the motor neuron-peripheral target circuit may provide therapeutic insight into damaged nervous tissue and diseases associated with motor neurons and nerve conduction.